Try elevating that pair of 90.0 ohm resistors across the filament supply with a voltage divider to ground off of the b+. Something like 40V ought to be ok.. 220K/43K with a 10uF 100V cap in parallel.
Also Ireland has 50Hz power, and full wave rectifier noise will be 100Hz. 60Hz/120Hz for USA and Canada..
Also Ireland has 50Hz power, and full wave rectifier noise will be 100Hz. 60Hz/120Hz for USA and Canada..
From my trials, what seems to work best is a pair of 100 ohm resistors coming from the cathode of one of the power tubes, to both sides of the 6.3v heater supply, and also an electrolytic cap from that point to ground.
If the preamp section has a separate heater supply, try the same thing there.
I'm no expert but this seems to work in most cases.
The other thing, as I recall that is effective, is a pair of resistors from the HV rail to ground, with a much smaller one to divide the last 30v or so, and connect one end of the heater AC to that junction. A filter cap is good here too.
Yes, most hum bucking pots I've seen are 100 ohms.
The hum is caused mainly, I believe, in the earlier sensitive preamp stages, as the AC heater supply forms a diode with the cathode, and that ends up as unwanted hum, if the heater sides are not at a higher potential (elevated) than the cathode offset to ground.
If the preamp section has a separate heater supply, try the same thing there.
I'm no expert but this seems to work in most cases.
The other thing, as I recall that is effective, is a pair of resistors from the HV rail to ground, with a much smaller one to divide the last 30v or so, and connect one end of the heater AC to that junction. A filter cap is good here too.
Yes, most hum bucking pots I've seen are 100 ohms.
The hum is caused mainly, I believe, in the earlier sensitive preamp stages, as the AC heater supply forms a diode with the cathode, and that ends up as unwanted hum, if the heater sides are not at a higher potential (elevated) than the cathode offset to ground.
Referring back to the crappy schematic I drew:
link to post 28
If I put in new tubes, do I need to do something about biasing or is this automatic? I can't really see anything to adjust except there are 2 trim pots connected to the grid or one half (I think) of the 6N1 tubes and back to ground
Fran
link to post 28
If I put in new tubes, do I need to do something about biasing or is this automatic? I can't really see anything to adjust except there are 2 trim pots connected to the grid or one half (I think) of the 6N1 tubes and back to ground
Fran
Hi all,
some good news to report. I found some 470uF 200V caps in an old PC power supply here and put them into the power supply (in the voltage doubler). Well, the hum has dropped significantly, to about half what it was, around 1mV on headphones and 2-3mV at speaker terminals! so good news...
I'm awaiting delivery of some 400V caps for the filter, hopefully they will make a difference too. I also have some parts coming to change to DC heaters.
Fran
some good news to report. I found some 470uF 200V caps in an old PC power supply here and put them into the power supply (in the voltage doubler). Well, the hum has dropped significantly, to about half what it was, around 1mV on headphones and 2-3mV at speaker terminals! so good news...
I'm awaiting delivery of some 400V caps for the filter, hopefully they will make a difference too. I also have some parts coming to change to DC heaters.
Fran
well,
I just had to let you guys know what happened next....
so I had the amp on for about 20mins on test after putting in the new caps. All was AOK, after 20mins hum on loudspeaker conns were 1.5mV and 2.1mV - not bad!! So I put the baseplate back on, moved her over to wher I normally keep it, turn it on and after maybe 30secs, it dies. 'm going "Oh sh!t". So I test the fuse in the amp, its ok. I test the fuse in the power lead - its OK. I bring it back to the test bench (AKA the kitchen table) and plug it into my test lead and it powers up! then dies again. So I can't figure this thing out and when I go to pull out the power cord, hey presto, it comes back again. Turns out there was a cold solder joint where the cable from the switch goes onto the terminal on the IEC plug. A quick solder job and its back up..... thats if it hasn't died whe I go back into listen again!
As this stage I've ahd the bottom off this thing more often than I've had hot dinners!
Fran
I just had to let you guys know what happened next....
so I had the amp on for about 20mins on test after putting in the new caps. All was AOK, after 20mins hum on loudspeaker conns were 1.5mV and 2.1mV - not bad!! So I put the baseplate back on, moved her over to wher I normally keep it, turn it on and after maybe 30secs, it dies. 'm going "Oh sh!t". So I test the fuse in the amp, its ok. I test the fuse in the power lead - its OK. I bring it back to the test bench (AKA the kitchen table) and plug it into my test lead and it powers up! then dies again. So I can't figure this thing out and when I go to pull out the power cord, hey presto, it comes back again. Turns out there was a cold solder joint where the cable from the switch goes onto the terminal on the IEC plug. A quick solder job and its back up..... thats if it hasn't died whe I go back into listen again!
As this stage I've ahd the bottom off this thing more often than I've had hot dinners!
Fran
OK,
I added in 2 more caps in parallel with the (filter?) caps - the 400V/120uF ones. I added in some 220uF/400V caps. The hum is now down to <1mV on one side and 1.1-1.2mV on the other and I can't get a reading at the headphone outs anymore. Hum is still there but is very faint now.
I tried changing the heaters to DC (1N5820 schottkys, 3300uF, 0R3, 3300uF) got 5.9VDC from the 6.3V winding but no further reduction in the last bit of hum. So I didn't bother going any further with that since fitting that into the chassis would have been a pain!
So I'm just waiting on new mil spec tubes and will report further.
To give an example of the level of the hum, the tape hiss on many recordings would be louder.
Fran
I added in 2 more caps in parallel with the (filter?) caps - the 400V/120uF ones. I added in some 220uF/400V caps. The hum is now down to <1mV on one side and 1.1-1.2mV on the other and I can't get a reading at the headphone outs anymore. Hum is still there but is very faint now.
I tried changing the heaters to DC (1N5820 schottkys, 3300uF, 0R3, 3300uF) got 5.9VDC from the 6.3V winding but no further reduction in the last bit of hum. So I didn't bother going any further with that since fitting that into the chassis would have been a pain!
So I'm just waiting on new mil spec tubes and will report further.
To give an example of the level of the hum, the tape hiss on many recordings would be louder.
Fran
Chinese...
This is the mini-version of this..:
http://www.diyaudio.com/forums/showthread.php?postid=1124955#post1124955
Arne K
This is the mini-version of this..:
http://www.diyaudio.com/forums/showthread.php?postid=1124955#post1124955
Arne K
Yes very similar, and maybe even the same PCB with different components. I see yours has a bridge rectifer whereas mine has a half wave voltage doubler.
Mine is sounding pretty good now, I'm using it mainly as a headphone amp but I have connected it up to speakers. It has more bass than my other vlave amp, an audio innovations amp. Probably would be warmer than it.
Are you happy with your el34 amp now since you made your changes?
Fran
Mine is sounding pretty good now, I'm using it mainly as a headphone amp but I have connected it up to speakers. It has more bass than my other vlave amp, an audio innovations amp. Probably would be warmer than it.
Are you happy with your el34 amp now since you made your changes?
Fran
Just a quick update. I got some russian 6P1P-EV and 6N1P-EV and have them in place now. Sound is a bit better, but not a huge difference bit better defined on bass and bit more clarity, but nothing thats screaming out at you. There is still a bit of hum/buzz but its pretty low level. I still think its something to do with the heaters. If I were to change to DC, should I still use a centre tap through 100R resistor to ground? Or elveate it off the B+ as suggested above?
On a side note, I've seen the guts of a few amps like mine recently. It seems that they all have the same PCB, but with some circuit differences depending on whether 6P1/EL84 or EL34 is used.
Fran
On a side note, I've seen the guts of a few amps like mine recently. It seems that they all have the same PCB, but with some circuit differences depending on whether 6P1/EL84 or EL34 is used.
Fran
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